On-board fuel inerting system

ABSTRACT

An inerting system is disclosed that is adaptable to inert the fuel tank of a vehicle, most typically an aircraft, that includes an oxygen detector to monitor the oxygen partial pressure of the vapors in the ullage (i.e., the overfuel) volume of the tank, a source of an inert gas (e.g., nitrogen) in valved communication with the ullage of the tank, and a detector for sensing the oxygen content in the ullage of the tank and controlling the flow of inert gas to the tillage to maintain that volume with a proportion oxygen that will not support combustion in the event of an ignition source or intrusion of another potentially explosive occurrence within said tank. A specific fiberoptic probe which enables monitoring oxygen content within the tank without introducing a source of electrical current within the tank is also disclosed.

[0001] This invention relates to a novel system for inerting combustibleand potentially explosive fuel supplies in vehicles such as aircraft.More particularly, this invention provides a novel nitrogen fuelinerting system that will fill the ullage (i.e., the vaporous volumewithin a fuel tank above the liquid fuel) of an aircraft fuel tank orother fuel tank with an inert gas that will not support combustion. Theinvention concomitantly provides a capability of actively monitoring theoxygen content of vapor and gas in the ullage of a fuel tank to maintainthe inert status of the over-fuel vapors and enable efficient dispensingof the inert gas.

BACKGROUND AND PRIOR ART

[0002] Since 1959, there have been a number of explosions of the centerwing tank on military and commercial aircraft reportedly resulting indeaths of as many as 550 persons. Those infrequent but continuingoccurrences involving fuel tank explosions are believed to have possiblybeen caused by unknown sources of ignition, possibly initiated byconductive wires exposed to the explosive fuel vapor/air mixture in thetank. These accidents have added impetus to the search for an effectivesystem for inerting flammable and potentially explosive vapors in fueltanks, particularly of aircraft. Inerting systems using halogen-basedgases have been known for use in military aircraft. But the use ofhalogen-based gases is not viable in commercial aircraft and in generalaviation because of their effects resulting in ozone depletion. Anon-halogen based system, moreover, would be advantageous in militaryapplications as well because of the more environmentally friendly natureof an inerting system based on a non-halogenic gas.

[0003] A brief discussion of the history of the problem of center fueltank explosions and of the growing interest in implementing an inertingsystem on commercial aircraft is found in Air Safety Week, Vol. 15 No.16, Apr. 16, 2001, “Fatal Explosion Highlights Hazard of FlammableVapors in Fuel Tanks.” The discussion in that article pointed out that,in particular, the center wing tank of commercial aircraft can reachhigh temperatures when the aircraft required to queue up on the hottarmac of an airport at summer temperatures, and that such conditionshave been linked to some explosions. The issue of Air Safety Week notedabove also discusses a number of nitrogen based inerting systems thatwere considered by the Aviation Rulemaking Advisory Committee at itsmeeting of Apr. 4, 2001. Included among those systems was a briefdiscussion of the system of the present invention.

SUMMARY OF THE INVENTION

[0004] Accordingly, this invention provides a nitrogen inerting systemfor potentially flammable and explosive fuel-vapor mixtures that existin fuel tanks, particularly in aircraft fuel tanks. However, it will beapparent that the systems of this invention are readily adaptable toinerting fuel tanks in any type of vehicle and hence this invention mayfind utility in various types of military vehicles or in vehiclesintended for operation in a hazardous environment or under othercircumstances where there is sufficient danger of fuel tank explosion orsufficient desire to negate such a possibility so as to justify theinstallation of the system. It is also apparent that the system of thisinvention could be also employed to inert a non-fuel tank environmentsuch as a cargo hold, or to provide a fire suppression system forselected areas of the aircraft or other vehicle, In such an instance,the nitrogen gas to inert an area might be released only after thepotential of fire is detected elsewhere on the vehicle. Hence, althoughthis invention will be explained herein in the context of an onboardnitrogen inerting system for an aircraft center wing tank, it will beunderstood that the system may be adapted to other fuel tanks or tomultiple fuel tanks on aircraft and other vehicles, and can further beadapted to create an inerted environment or supplemental firesuppression system in a volume other than a fuel tank. Most likely thissystem will be used principally as a fuel tank inerting system, will besimultaneously adapted additionally to provide a fire suppression systemin other regions of the aircraft, such as a cargo hold.

[0005] In particular, the instant invention provides a system thatprovides an inert blanket of a nitrogen rich gas atmosphere to a fueltank or other environment so as to inert that environment from theprospect of explosion. In a further aspect of this invention there isprovided a system with safe and effective monitoring of the oxygenconcentration (preferably measured as oxygen partial pressure) in theatmosphere in a tank or other area to be inerted such that nitrogen canbe metered efficiently from a liquid nitrogen source to maintain theatmosphere at a desirably low oxygen concentration so that the potentialof explosion is minimized or eliminated. Temperature of the tank canalso be monitored further to assess the potentially explosive characterof the tank contents. There is further provided an effective probe whichcan effectively monitor the oxygen content of the atmosphere within afuel tank, and which can accomplish that monitoring function accuratelywithout introducing a dancer that the probe itself can be a source ofsparking that could initiate an explosion. In a special embodiment, theprobe is a passive fiber optic probe that can be extended into the fueltank or other environment to monitor oxygen content and provide a basisfor controlling oxygen concentration. Because, in the particularembodiment of this invention, the output of the fiber optic probeindicative of oxygen concentration is temperature dependent, there isalso provided herein a shroud or device to maintain the probe at a knownuniform temperature in order to assure an accurate oxygen measurementcan be obtained.

[0006] The system of the instant invention utilizes an on-board liquidnitrogen supply in the form of a dewar or similar liquidnitrogen-holding container which is suitably insulated to maintain theliquid nitrogen at reduced temperature for a long period. The containerdesirably is sized to provide a sufficient supply of nitrogen to blanketthe fuel tank, for example the center wing tank or multiple fuel tanksof an aircraft, preferably several times over as will be explainedbelow. The on board nitrogen supply is in valved fluid communicationwith the fuel tank or tanks or other environments to be inerted, mosttypically the aircraft's center wing tank. A passive fiber optic probethat extends into the tank monitors the oxygen partial pressure withinthe tank. The output from the probe can be fed to a spectrometer orsimilar instrument that is adapted to translate the analog output of theprobe to a digital signal which can be fed to a suitable controller,such as a microprocessor-based controller. The microprocessor can thencontrol valves through which nitrogen flows from the onboard nitrogensource to feed nitrogen to the tank as needed to maintain the over fuelatmosphere in an inerted condition. The apparatus can control that flowand maintain an inert condition by monitoring a signal from the probeand using software adaptations that will be understood by those skilledin the art to enable the microprocessor to control the nitrogen contentin the ullage of the tanks.

[0007] The accurate probes contemplated by this invention provide anoutput signaling oxygen partial pressure that is temperature dependent,and hence the invention also provides a receptacle or shroud to providea stable temperature environment which can be used in the case of asystem which is subject to the widely varying temperatures the will beencountered by a commercial aircraft. That is, the invention provides aheated receptacle or well to encase the probe extending into the fueltank to maintain the probe at a sufficiently constant temperature toassure accurate oxygen readings by the probe. The fuel tank is suppliedwith a suitable relief valve that will expel the over-fuel gas-vapormixture as nitrogen is metered from the on board supply to the tank. Theoverall system, however can be operated at relatively low peak pressuresof 20 psi over ambient or less. The system of this invention accordinglyovercomes the disadvantages of some suggested systems that requirecompressors on board to effect nitrogen separation from air.

[0008] Accordingly, in summary, the instant invention provides anon-board fuel inerting system for a vehicle subject to electricalsparking or other intrusion of potentially explosive occurrences withina fuel tank of the vehicle which comprises: (a) an oxygen partialpressure detector maintained in contact with the vapors in the ullage ofthe fuel tank; (b) a source of inert gas maintained on-board the vehiclein valved communication with the ullage volume of the fuel tank; and (c)a controller responsive to the detection of a partial pressure of oxygenwithin said ullage volume that is higher than some predetermined levelto cause inert gas to flow into the ullage volume from said source toestablish the oxygen partial pressure below said predetermined level. Inthe preferred embodiments, the system is adapted to an aircraft usingnitrogen gas as the inerting agent and using a fiberopticoxygen-detecting probe which makes it possible to detect oxygen partialpressure within the tank without exposing the ullage volume toelectricity and an additional source of sparking.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The present invention will be further exemplified and explainedwith reference to the accompanying drawings which are as follows:

[0010]FIG. 1 is a schematic diagram of a system in accordance with thisinvention disposed to maintain an inerted atmosphere in the center wingtank of a commercial aircraft.

[0011]FIG. 2 is sectional view of a shroud or receptacle for a fiberoptic probe in accordance with the present invention, the well orreceptacle being adapted to maintain the probe at a relatively constanttemperature to assure suitably accurate readings of the oxygen contentof the tank being inerted.

[0012]FIG. 3 shows a baffle arrangement that can be employed as analternative to the arrangement shown in FIG. 2 to create an atmospherethat will accurately reflect the oxygen concentration in the tank beinginerted, yet afford an opportunity to shield the probe from beingcontacted by liquid fuel.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

[0013] Referring to FIG. 1 there is shown schematically a fuel tank 11having fuel in the lower portion thereof. The overfuel or ullage volumeof the tank 12, also known as ullage, would accordingly be typicallyfilled with an air-fuel vapor mixture which would very likelycombustible and explosive if the fuel vapors were combined with ambientair. Nitrogen gas line 15 is capable of conducting nitrogen to the uppermost portion of the tank, and is in valved communication with a nitrogendewar 13. Any suitable insulated container may be used, however, a dewarcontainer of the type typically used to store liquefied gases ispreferred. Such container may have, for example, a double wall withevacuated space between the walls and reflective or silvered surfaces.Although any suitable insulative type container for storing liquefiedgas may be used, the nitrogen container shall be referred to as a dewarherein.

[0014] In accordance with a particular aspect of the invention thesupercooled liquid nitrogen is maintained in a dewar or insulatedcontainer capable of holding about 100 gallons of liquid nitrogen. Sucha container is capable of generating about 9300 cubic feet of nitrogengas at approximately atmospheric pressure. Such an amount of gas wouldbe enough to fill the center wing tank of a 747-type aircraft (which isabout 2100 cubic feet by volume) more than three times over,Accordingly, the use of such a dewar even on such a large aircraft suchas a 747 commercial jetliner would mean that the center wing tank couldbe inerted for as many as three flights on one supply of liquid nitrogenprovided to the dewar.

[0015] A valve 17 such as a solenoid valve is provided in line 15between the dewar 13 and the fuel tank 11. Valve 17 is controlled by thecontrol system hereinafter discussed to feed nitrogen to the overfuel orullage (ullage) volume of the fuel tank 11 when it is detected that theoxygen 9 concentration in the overfuel volume has climbed sufficientlyhigh to make the ullage vapor mixture potentially explosive.

[0016] The nitrogen dewar 13 is provided with a suitable fill connection14 that terminates on the external skin of the aircraft of at anothersuitably accessible point on the aircraft to enable airport supportpersonnel to refill the dewar from a liquid nitrogen supply truck or thelike as needed.

[0017] Additional lines such as 15 can communicate from the nitrogendewar to other fuel tanks on the aircraft. However, it is pointed outthat the center wing tank of the aircraft is likely the prime candidatefor this inerting operation.

[0018] Level detector 29 is an apparatus that detects the level ofliquid nitrogen in the dewar. Output from the level 29 is communicatedto the microprocessor-based controller signaling the need for fillingthe dewar when the nitrogen supply is depleted.

[0019] Line 21 is an alternate nitrogen gas carrying line that can be incommunication with other portions of the aircraft, for example, cargohold areas where fire may be a potential problem. Under suchcircumstance, control valve 25, for example another solenoid valve, andpressure regulated valve 23 can be operated to permit nitrogen gas toflood the regions in communication with line 21 thereby suppressing thefire hazards in those areas. The pressure regulator 23 may be aself-regulated valve that opens when the pressure differential betweenthe upstream and downstream ports of the valve exceed the valve setpoint which may be adjusted to desirable operating pressures as will beunderstood by those skilled in the art. Inert gas is not providedthrough the cargo hold areas on a continuous basis, but is only providedif and when some potential fire hazard in those areas is detected andsolenoid valve 25 is opened by the control system. Accordingly, it mayor may not be necessary to deploy an oxygen detecting probe as discussedbelow in those cargo hold or other non-fuel tank regions, since thehazard in those areas will likely have been signaled by other thanoxygen partial pressure. However, it will be apparent to those skilledin the art that deployment of an oxygen probe in any area for whichoxygen may be supplied can be effective to inform the controller of theoxygen partial pressure. An oxygen detecting capacity may be deployed inthe cargo hold area utilizing the probe discussed below and the constanttemperature well or receptacle that maintains such probe at asufficiently uniform temperature so that the probe is capable ofaccurately detecting oxygen partial pressure. It must be understood thatan aircraft is subjected to a wide range of temperatures as it proceedsthrough the cycle of takeoff to high altitude and return to an airportand in accord with this invention it is desirable to maintain the oxygenmeasuring probes at relatively uniform temperature.

[0020] Turning now to the region of the fuel tank, the aspects of thecontrol apparatus of the inerting system of the present invention willbe discussed. In this context, it should be noted that lines which aresolid on FIG. 1 indicate that the line is capable of transmitting fluidsuch as nitrogen gas or other gas streams as will be discussed herein.Lines that are shown as dotted indicators are intended to indicate thatthe lines do not carry fluid but are for communication and controlpurposes. Such lines capable of providing signals to detect pressures,temperatures and the like and feed those signals to the microprocessorcontrol apparatus in accordance with the present invention.

[0021] Normally, the inerting system of this invention can operate atrelatively low pressures, as low as about 20 or 25 psi over ambientpressure. Accordingly, pressure regulator valve 27 is a pressure reliefvalve that relieves any accumulation of pressure in the dewar and ventsthe inert gas through line 26 into the overfuel volume within the fueltank. The pressure set point in valve 27 is typically higher than thesetting in pressure regulator 23 since valve 27 is a relief portdesigned to relieve and valve 23 feeds inert gas to the fuel tank whenthere is at least some minimal pressure drop between the dewar and thefuel tank being inerted. This information is fed back to microprocessorcontroller 41, which may be a personal computer-type controller, so thatvalve 17 can properly be operated to meter nitrogen to the fuel tankwhen desired. The liquid nitrogen in the dewar can provide sufficientpositive pressure to flow the nitrogen from the dewar to the tank whenvalve 17 is open and the pressure drop between the dewar and the tankexceeds the setting of valve 23. Tank 11 is preferably also providedwith a vent line 42 that will release the overfuel gases of any typeinto the atmosphere when the pressure of those gases in the fuel tankexceeds a desirable maximum. If desired a valve may be inserted in thatvent line to establish that pressure differential.

[0022] One key aspect of the system of the instant invention is oxygenprobe 51 that extends through constant temperature receptacle 59 intofuel tank 11. Probe 51 serves to monitor the oxygen concentration asoxygen partial pressure in the fuel tank at all times. Simultaneously,the within the fuel tank is monitored by temperature sensor 31 which maybe a thermocouple or other typical temperature sensor well know to thoseskilled in the art. Hence the microprocessor controller 41 is suppliedwith both temperature information and oxygen partial pressureinformation concerning the tank on a continuous basis. When the controlsystem senses that oxygen partial pressure within the tank isapproaching a potentially hazardous level in view of tank temperature,the control system can activate the nitrogen supply through the valvesdiscussed above and can feed nitrogen to the overfuel volume of thetank, expelling gas through the tank vent until the desired low level ofoxygen partial pressure is achieved.

[0023] The oxygen probes used in accordance with this invention arefiberoptic sensors which use chemical fluorescence which can begenerated at the sensor tip to measure the partial pressure of oxygen.Hence, the sensors used in accordance with this invention do not includeany current carrying wires extending into or near the center wing tankor any other fuel tank which is being monitored for the development of apotentially explosive air/fuel vapor mixture. The fiberoptic sensors areused in combination with a spectrometer to achieve real time measurementof oxygen partial fresh air in the ullage mixture over the fuel in tank11. Optic sensors used in accordance with this invention are adaptedfrom optic sensors presently used, for example, for respirationmonitoring. These sensors typically consist of a reflective probe with aspectrophotometer or suitable monochromator 43 for wavelength specificanalysis.

[0024] The probes which are illustrated as item 51 in FIG. 2 areavailable commercially. The probes operate by having optical fiber carryan excitation light produced, for example, by a blue LED to a sol-gelthin film coated on the membrane at the probe tip that extends into theatmosphere being monitored for oxygen partial pressure. Fluorescencegenerated at the tip is collected by the probe and carried by opticalfiber to the detector (the spectrometer or monochromator 43). Whenoxygen in the sample being monitored diffuses into the coating, itquenches the fluorescence. The degree of quenching correlates to oxygenpartial pressure level. The oxygen pressure measurement can be obtainedin either a dry or wet gas environment. Hence by mounting the probesthrough the fuel tank wall so that the tip is exposed to the vaporousmixture in the fuel tank ullage, the probes can effectively provide ameasurement of oxygen partial pressure within the tank.

[0025] An entirely suitable optic oxygen sensor which may be used inaccordance with this invention is the FOXY Fiber Optic Oxygen Sensorcurrently manufactured and offered for sale by Ocean Optics, Inc, ofDunedin Fla. These FOXY Fiber Optic Sensors use the fluorescence of aruthenium complex in a sol-gel to measure partial pressure of oxygen. Abrief description of the mode of operation of these optic sensorsfollows:

[0026] First, a pulsed blue LED located in the spectrometer 43 sendslight having a wave length of about 475 manometers to an optical fiber.The optical fiber line 55 carries the light to the probe, the distal endof which is exposed to the atmosphere to be measured. The distal end ofthe probe tip consists of a thin layer of hydrophobic sol-gel materialthat extends within the fuel tank being monitored for oxygen content. Aruthenium complex is trapped in the sole gel matrix at tip 53effectively mobilized and protected from water. The light from the LEDcommunicated over fiberoptic fiber lines to the probe tip excites theruthenium complex which fluoresces, emitting energy at about 600manometers wave length. If the excited ruthenium complex encounters anoxygen molecule, there is a decrease or quenching of the fluorescentsignal. The degree of quenching correlates to the level of oxygenconcentration or oxygen partial pressure in the thin film, which is indynamic equilibrium with the oxygen in the sample, in this case thevolume 57. The energy is collected by the probe and carries through thereturn light conductive fibers in fiberoptic line 55 to the spectrometer43. An A-D converter within spectrometer 43 converts this analog data todigital data, which is fed to a microprocessor, for example of the typein a personal computer and can then be displayed using suitablesoftware. Ocean Optics provides such software as OOISENSORS software.

[0027] Temperature, however, is known to affect the fluorescence decaytime, the fluorescence intensity, and the collisional frequency of theoxygen molecules with the fluorophore in the tip of probe 51.Temperature also affects the diffusion coefficient of oxygen and thesolubility of oxygen in the samples. Accordingly, it is necessary forbest results to maintain the sample at a constant temperature thatvaries no more than about +/−3° Centigrade, preferably no more than+/−1° Centigrade. In view of the wide temperature variations to whichcenter wing tanks are subject, it is highly desirable than when oxygenprobes such as those preferred by this invention are used, that they bedeployed into the center wind tank using a shroud or receptacle thatenables maintenance of the probe under temperature conditions that aresufficiently uniform to assure accuracy of the oxygen partial pressuremeasurement. Uniform temperature of the probe is achieved using a shroudor uniform temperature environment which can use temperature controlledgas or an electric heater to maintain the desired uniform temperature.Shown in FIG. 1 is a source of temperature-controlled gas 33 which inthe illustrated embodiment may be a manifold containing for examplewaste exhaust gases from the aircraft engine. Line 67 which is suitablyinsulated conducts the temperature controlled gas from the source 33 toshroud 59 as will be more particularly discussed below in connectionwith the shroud embodiment. A temperature sensor 83 senses thetemperature at the furthest point of oxygen sensor 51 and regulates thattemperature by regulating the operation of a suitable proportional valve36 which can be controlled to mix gas from the temperature controlledsource and ambient air to achieve the desired gas temperature input tothe shroud. Suitable valves are those sold under the Sentronic trademarkby Asco Pneumatic Controls of Fort Mill, S.C.

[0028] Fiberoptic probes which may be used to determine oxygen partialpressure in fuel tanks using the apparatus of this invention are furtherdescribed in the following articles which are incorporated herein byreference:

[0029] 1. Krihak, M. et al., A Highly Sensitive, All Solid State FiberOptic Oxygen Sensor Based on the Sol-Gel Coating Technique, ElectronicsLetters 1996, Vol. 32, No. 3.

[0030] 2. Wang, W. et al., Applying Fiber Optic Sensors for MonitoringDissolved Oxygen, Sea Technology, March 1999, Vol. 40, No. 3, pp. 69-74.

[0031] 3. Krihak, M, et al., Fiber Optic Sensors Based on the Sol-GelCoating Technique, Chemical, Biochemical and Environmental Fiber SensorsVIII, 1996, Vol.2836, pp. 87-98,

[0032] 4. Sharihari, M. R. et al, Ormosil Thin Films For ChemicalSensing Platforms, Chemical, Biochemical and Environmental Fiber SensorsIX, Vol. 3105, pp. 40-51.

[0033] These article represent disclosures of using these sensitivefiber optic probes and those of skill in the art will understand howthese disclosed probes may be adapted to determine oxygen partialpressure in the center wing tank environment in accordance with thepresent invention.

[0034] In accordance with this invention, it may be desirable to extendmore than a single probe into the center wing tank. For both safety andredundancy purposes, it is preferred to use at least two probes toaccomplish effective monitoring of a single fuel tank.

[0035] The fiberoptic sensors of Ocean Optics, Inc. are specifically theFOXY Fiber Optic Oxygen Sensors may be used with the Ocean OpticsLES-450 Blue LFD pulsed light source and the ST2000 MiniatureFiberoptics Spectrometer adapted for avionics use.

[0036] The signals indicating oxygen partial pressure are fed back tothe spectrometer 43 as indicated. The spectrometers of Ocean Optics areadapted to operate using suitable software available from the company ina personal computer environment. Accordingly, the Ocean Optics systemusing fiberoptic probes provides a ready means of generating a digitalsignal indicating oxygen concentration within tank 11 for amicroprocessor, e.g., a personal computer 41, which can be suitably thenprogrammed to control and monitor nitrogen flow to the fuel tanks sothat oxygen partial pressure does not exceed desirable levels. The datacollected by the p.c. concerning oxygen concentration is alsocontinuously fed to a flight data recorder 47 or “black box” typicallymaintained on commercial aircraft in the United States to enabletracking of malfunctions of the system after the fact. A cockpit statusdisplay light 45 with or without a display giving a current readout ofoxygen partial pressure signals (not shown) to the pilot that the fuellevel oxygen monitoring system is active and in operation when that isthe case. Activation of the cockpit status light display will signalthat the system is operative or inoperative, warning the pilot of thepotential malfunction or nonresponse by the inerting system, and isgenerally a preferred status indicator for use with this system,although a constant readout of oxygen partial pressure may also beprovided.

[0037] As indicated above, the fiber optic probes of the typerecommended for use with the present invention must be kept within aband of temperature to assure accurate readings. The tolerance fortemperature variation will depend upon the probe chosen andmanufacturer's recommendations with respect to such matters should beheeded. The constant temperature environment of the probe is maintainedby mounting the probe within an insulated receptacle or shroud that isactively maintained at an elevated temperature that is preferably at theupper end of the temperature range that the aircraft will be anticipatedto encounter (which of course will be higher during summer seasons thanwinter). The shroud or receptacle may be heated by heated gas from, forexample, the aircraft engines as illustrated below or using an electricresistance heater in order to keep the probe at the desirable constanttemperature. Such a receptacle or shroud is shown in FIG. 2.

[0038] Referring now to FIG. 2, they are showing a gas-heated receptacleor shroud 59 which centrally mounts the fiber optic sensor 51. Overall,the shroud comprises a cylindrical member having a bore through whichthe fiberoptic sensor 51 may be centrally mounted using gas tighthermetically-sealed bushings such as 58 and 58 a which may be plastic ormetal in composition. Overall, the cylindrical shroud is comprised ofseveral concentric regions, the outermost of which 54 is a vacuumchamber which serves to insulate the inner portions of the shroud fromoutside temperatures via conduction or convection. Concentrically,within the outermost annular vacuum chamber, which is sealed uponconstruction of the shroud, is a second concentric chamber adapted toreceive heated gases, for example heated waste gases from the aircraftengines though heated gas line into port 65. Port 65 communicates withannular concentric chamber 61 into which the heated gases flow. Ports 63permit the gas to flow from the chamber 61 to the central bore 65 of theshroud which surrounds the fiber optic sensor. A hermetically sealedfeed through bushing 58 a at the opposite end of the shroud centers theposterior portion of the probe. Hence, heated gas such as air, nitrogenor engine exhaust cases can be flowed through the heated air region, andthence into the bore to bathe the probe 51 with uniform temperature gas.Generally the probes in accord with this invention may be calibrated andoperated at temperatures that represent the high end of the temperaturerange to which the center wing tank can be subjected during preflightdelays on the tarmac of a takeoff line, generally as high as about120-140° Fahrenheit. The reason for this is that in many flights, thehighest temperature to which the fuel will be subjected is thetemperature on the runway either in summer or in winter. Accordingly,the system should be adjustable so that most the maximum targettemperature at which oxygen partial pressure may be measured willgenerally be from about 30 to 40° F. in winter to 130-140° F. in summer.When programmed to operate at a suitable temperature within that range,the probe can be maintained at a temperature by heating the probe withhot gas or using an electrical resistance heater to remain accuratethrough the entire range of temperatures that the center wing tank canbe reasonably expected to encounter. Hence in summer the planes may bestandardized to measure oxygen partial pressure at 135° F.

[0039] When the probe is mounted in the fuel tank, it is desirable thata sensing and measuring portion of the probe extends to within the fueltank volume. A heat-insulating layer 50 may be provided if desired whichmay be rigid plastic foam surrounds the shroud. Layer 50 can seal theprobe to the fuel tank using a compression fitting in any suitable placein the anterior region of the shroud for example at the region indicatedas 71 on FIG. 2. It will be noted that the shroud extends beyond the end53 of probe 51 to create a volume 57 which can be more effectivelyheated by the shroud. The gas within volume 57 will have the same oxygenpartial pressure as the tank into which the probe extends. A gaspermeable shield 69 which may be a metal screen or a gas permeable foamor fabric structure surrounds volume 57 and protects the probe frombeing contacted by liquid fuel within the tank while assisting instabilizing the temperature of volume 57. The double arrow signifies theready gas permeability of shield 69.

[0040] Referring to FIG. 3 in which similar numbers are used todesignate similar shroud parts, there is shown an alternative bafflestructure that can be used to define a volume 57 a which can beeffectively heated by the shroud 59. The shroud is compression-fitthrough the wall 48 of the fuel tank being monitored. A baffle systemcomprising a first frusto-conical member 73 which concentrically mountsa conical baffle 75 with struts 77. The baffle system generally protectsthe tip of the probe from being contacted by fuel, yet permits the gasfrom the tank to freely circulate into volume 57 a as indicated by arrow81. Slots around the periphery of member 73 in the region of 79precludes that liquid fuel will become trapped within the baffle system.

[0041] In operation, it is preferred to operate the fiber optic probesat a temperature at about 120-135 degrees Fahrenheit under summerconditions, but as low as 45-60 degrees Fahrenheit during winter. Therange will depend on the maximum temperature anticipated. The probes ofthis invention can deliver accurate oxygen partial pressure measurementthroughout such a range and indeed over even a broader range oftemperatures. If such is the desired temperature of operation, it willbe understood that a heated gas source may be continuously operated andcontain a flow of gas at temperature to the heated area and leading tothe heated air volume of the shroud. The use of a guard around theoxygen-measuring tip 53 of the probe in the form of permeable shield ora baffle system which can be configured by those of skill in the art canprovide a volume that will equilibrate readily temperature-wise with thetemperature of vapor and will substantially guard against the tip 53being contacted by liquid fuel in the tank. The use of such a guard willtherefore enable the measurement of oxygen content in the immediatevicinity of the oxygen-measuring region of the probe which is at tip 53of the probe to be closely representative of the oxygen content of theentire tank, while providing protection against contact with liquidfuel. As indicated above, heated nitrogen can be used if desired,although the shroud can be suitably constructed to be reasonably surethat leakage of air will not pass into the fuel tank.

[0042] The operation of the overall system will be explained in thecontext of the probe and shroud discussed above. It has been generallyrecognized that the likely greatest potential danger of center wing tank(and other fuel tank explosions) probably occurs during the highertemperature seasons when aircraft spend time on the tarmac and fuel tanktemperatures are in excess of 85 to 100° Fahrenheit or higher. Underthose conditions, the system of this invention will flood the fuel tanksbeing controlled with nitrogen to minimize the explosive character ofvapor in the ullage of those tanks. As the aircraft climbs out andtemperature increases, the temperature of the fuel will decrease ataltitude and nitrogen will flow into the tank through line 26 becausepressure in the tank will decrease as the temperature and volumedecreases. Line 42 is equipped with a check valve which permits outflow,but not inflow of atmospheric air, when the pressure in the tank isseveral psi higher than atmospheric air. Hence nitrogen will flowthrough self-regulated valve 27 to the tank when the pressure of gas inthe ullage of the tank decreases by to whatever predetermined level ispermitted by the valve. Although not shown, valve may be adapted to beunder the control of controller 41 as well, although the control of flowto tank 11 through controlled valve 17 may be sufficient. During theentire flight, moreover, the oxygen partial pressure in the ullage ismeasured by the oxygen probe 51 in the receptacle or shroud 59 and theinformation about oxygen content of the ullage volume is read byspectrometer 43 so that nitrogen can be fed through line 15 controlledby solenoid valve 17 to the tank to maintain the desired low partialpressure of oxygen in the tank. As indicated above, the gas pressure intank 11 is slightly above ambient, and when nitrogen flow occurs throughline 15, the oxygen rich gas in the ullage volume is purged through ventline 42. Heated gases from source 33 (e.g, engine waste gas) under thecontrol of controller 41 through spectrometer 43 which can modulate theflow of heated gas through valve 36 in response to the temperaturesensed by temperature sensor 34, maintain the shroud at a relativelyconstant temperature. The shroud temperature control may be otherwisemaintained using an electric resistance heater with resistance wiresembedded in a solid core and surrounding the probe 51, but otherwiseconforming to the general configuration shown in FIG. 2. Hence, thesystem can provide an efficient way to maintain a low oxygen content inthe tank at all times during a flight from taxi to landing.

[0043] Adapting the system to the monitoring of cargo holds will also beapparent to those of skill in the art. The cargo holds may be equippedwith probes in accordance with this invention or with more conventionalsmoke detectors, response to which would cause the system to flood thehold volume where any difficulties might be detected with nitrogen.

[0044] The foregoing discussion of specific embodiments of the instantinvention should not be considered limiting, and those of skill in theart will understand that variations in the configuration of componentsmay be made without affecting the spirit and scope of the inventiondisclosed. For example, the valving system that is employed to conductnitrogen or other inert gas to the tank may be varied as will beapparent. As another example, the uniform temperature shrouds might belocated outside the tank being monitored with samples being taken fromwithin the tank and conducted to an oxygen detectors or multiple oxygendetectors maintained in a uniform temperature environment. Under suchconditions sampling may be continuous to multiple detectors or samplingmight be sequential of the tanks using one or a few detectors tominimize uniform temperature requirements. If the detector unit ismaintained in the pressurized and temperature-controlled environment ofthe passenger cabin or cockpit of an aircraft, it would be possible toconduct samples continuously or intermittently from the tanks beingmonitored to a central oxygen-content measuring station in, for example,the cabin where an oxygen detector in accord with this invention wouldmaintain a constant watch on oxygen partial pressure within the fueltanks to maintain the vapors in a relatively non-explosive condition.Such an arrangement would simplify the effort needed to maintain theprobe at a uniform temperature since cabin temperatures do not fluctuateas widely as tank temperatures during flight. Finally, it will beapparent that the instant system could be readily adapted to renderinert the fuel tank of another type of vehicle, such as a combat vehiclewhere the fuel tank is subject potentially explosive occurrences in thetank. Hence the disclosure herein which is directed to those of skill inthe art should not be considered limiting but should be consideredexemplary and the invention shall be defined by the claims.

What is claimed is:
 1. An on-board fuel inerting system for a vehiclesubject to electrical sparking or other intrusion of potentiallyexplosive occurrences within a fuel tank of the vehicle which comprises:(a) an oxygen partial pressure detector maintained in contact with thevapors in the ullage of the fuel tank; (b) a source of inert gasmaintained on-board the vehicle in valved communication with the ullagevolume of the fuel tank; and (c) a controller responsive to thedetection of a partial pressure of oxygen within said ullage volume thatis higher than a predetermined level to cause inert gas to flow into theullage volume from said source to establish the oxygen partial pressurein said volume at a lower level.
 2. The system of claim 1 wherein saidinert gas is nitrogen.
 3. The system of claim 1 wherein the said sourceof inert gas is a container maintaining such inert gas in a liquidcondition.
 4. The system of claim 1 wherein the oxygen partial pressuredetector is a fiberoptic detector which detects oxygen partial pressureby detecting the effects of oxygen on the fluorescence of a materialmaintained on the probe and subjected to contact with the ullage volumevapors.
 5. The system of claim 5 wherein the on-board inerting system isaboard an aircraft and wherein oxygen partial pressure probe ismaintained in a receptacle capable of being maintained at substantiallyuniform temperature throughout a flight of the aircraft.
 6. An on-boardfuel inerting system for an aircraft subject to electrical sparking orother intrusion of potentially explosive occurrences within a fuel tankof the aircraft which comprises: (a) an oxygen partial pressure detectormaintained in contact with the vapors in the tillage of the fuel tank;(b) a source of inert gas maintained on-board the vehicle in valvedcommunication with the ullage volume of the fuel tank; and (c) acontroller responsive to the detection of a partial pressure of oxygenwithin said ullage volume that is higher than a predetermined level tocause inert gas to flow into the ullage volume from said source toestablish the oxygen partial pressure in said volume at a lower level.7. The system of claim 1 wherein said inert gas is nitrogen.
 8. Thesystem of claim 7 wherein the said source of inert gas is a containermaintaining such inert gas in a liquid condition.
 9. The system of claim7 wherein the oxygen partial pressure detector is a fiberoptic detectorwhich detects oxygen partial pressure by detecting the effects of oxygenthat exists in the vapor within the ullage volume on the fluorescence ofa material maintained on the probe and subjected to contact with theullage volume vapors.
 10. The system of claim 10 including (d) a uniformtemperature receptacle surrounding the portion of the fiberoptic probein contact with said ullage volume to stabilize the temperature of theprobe at a substantially constant temperature throughout a flight. 11.The system of claim 11 wherein the uniform temperature receptaclesurrounds the oxygen-measuring region of the probe to create a volume ofrelatively constant temperature.
 12. The system of claim 11 wherein theprobe extends into the ullage volume of the aircraft fuel tank and saiduniform temperature receptacle is a heated shroud surrounding saidfiberoptic probe.
 13. The system of claim 13 wherein theoxygen-measuring region of said probe is surrounded by a guard thatpermits entry of vapor in the ullage volume to exist in proximity to theoxygen-measuring region of the probe, but protects the probe fromcontact with liquid fuel within the tank.
 14. An on-board fuel inertingsystem for the center wing fuel tank of a commercial aircraft subject toelectrical sparking or other intrusion of potentially explosiveoccurrences within that fuel tank which comprises: (a) an oxygen partialpressure detector maintained in contact with the vapors in the ullage ofthe fuel tank; (b) a source of liquid nitrogen gas maintained on-boardthe vehicle in valved communication with the ullage volume of the fueltank, said source containing an amount of gas sufficient double theamount needed to flood the ullage of said fuel tank; and (c) acontroller responsive to the detection of a partial pressure of oxygenwithin said ullage volume that is higher than the proportion that wouldsupport combustion to cause inert gas to flow into the ullage volumefrom said source to establish the oxygen partial pressure in said volumeat a lower level.